The instruments here contemplated, in which the strings have major vibratory portions in a plane generally parallel to the surface of a body such as a soundboard to which they are anchored under tension, include not only those of the neck type (guitars, violins etc.) but also pianos and the like. In such instruments it has already been proposed to provide electroacoustic pickups of the piezoelectric type (see, for example, U.S. Pat. Nos. 3,712,951, 3,530,228 and 3,049,958) as well as photoelectric transducers (U.S. Pat. No. 3,733,953). The use of a strain gauge in a vibraphone, xylophone or marimba has also been suggested (U.S. Pat. No. 3,684,814).
As disclosed in my above-identified copending application, a bridge member separating a set of strings from a resonant instrument body can be divided by slots into a number of sections each individually supporting a single string or a group of strings whose vibrations are to be picked up by one or two strain gauges carried on the respective section, the direction of maximum sensitivity of these strain gauges being perpendicular to the body surface and to the longitudinal direction of the strings. Such a strain gauge is highly sensitive to transverse vibrations of the strings but will not respond to oscillations of the resonant body at its various natural frequencies as induced in that body by the impinging sound waves. In another copending application, Ser. No. 123,443 filed Feb. 21, 1980 and now abandoned, I have disclosed circuitry particularly designed for the faithful reproduction of the resonance frequencies of such a body which generally lie in a range of up to about 5 KHz.
Aside from the periodically varying pressures exerted by a vibrating string upon a bridge or a section thereof perpendicular to the supporting body surface, however, there are also generated shear stresses parallel to that surface which result from the periodic changes in the effective length of the string (referred to in the art as "speaking length"). Since each transverse excursion of the string from its normal position foreshortens that effective length, these longitudinally oriented shear stresses have a periodicity which is twice that of the transverse vibrations giving rise thereto. Thus, the spectrum of the shear stresses is rich in even harmonics of the fundamental string frequency and its harmonics which, however, are not picked up by transversely oriented strain gauges. Theoretically, these even harmonics could be sensed by differentially connecting the outputs of a pair of strain gauges disposed on opposite parallel faces of a bridge section as disclosed in my prior application Ser. No. 935,916, yet this would require a careful balancing of the transverse forces acting upon these two faces.